Currently, pressure sensors are used in a number of applications where helium is used. Helium is sufficiently small a molecule that it is routinely used to detect leaks in systems, including slow leaks through glass to metal seals. This represents a problem for the standard pressure sensor built with a silicon micro-machined transducer when the die is bonded to glass for absolute pressure measurements. Helium diffuses through the glass and fills the reference chamber, thereby giving the impression of a zero shift in the output as a function of exposure to helium. Ultimately, the cavity will become filled with helium at a pressure equal to the helium pressure outside the sensor. The diffusion rates appear to be somewhat exponential. Typically, the time constant associated with this filling process is in the range of 50 to 100 hours. Thus in that time period, the vacuum reference chamber will fill with 63% of the outside helium pressure.
To overcome this, people have used fusion bonding of two silicon wafers together. The diffusivity of helium through silicon is such that the leakage rates are not measurable.
The disadvantage of the fusion bonding process is that it is not as often used as the anodic bond process for pressure sensors and the metal has to be put down after the fusion bond, meaning special processing. Moreover, while glass can be easily visualized for defects in the bond region, the fusion bond does not allow this. A further disadvantage is that the fusion bonded wafer approach cannot be purchased from a foundry service ready for micro-machining. Instead, the wafers have to be inventoried without metalization awaiting both fusion bonding and electrical testing.
All of the fusion bonding advantages could be overcome with a viable solution to helium leak in the conventional anodically bonded glass-silicon pressure sensors.
The present invention provides glass with metal deposited on it in a pattern which is slightly larger than the cavity area of the corresponding silicon wafer. The metal layer has a thickness such that the glass can deform sufficiently to allow bonding of the glass to the silicon. The metal is then compressively bonded to the silicon in a rim area around the edge of the cavity. The metal provides a barrier to helium, which can leak through the glass, but is stopped by the metal barrier.
In a preferred embodiment, a metal layer of between 600 and 1200 Angstroms is used, with either aluminum or titanium being the metal. An overlap beyond the cavity between 25 and 50 microns is preferred.
For further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.